This might be of interest:
Threema has just published their source code.
Source code on GitHub: https://github.com/threema-ch/threema-android
In case you wonder how a paid app can be open source:
The Threema apps are subject to the GNU Affero General Public License version 3. More details can be found in the source code repositories.
Please note that even though they may be compiled and modified freely, the Threema apps are still paid apps. An anonymous license check prevents the creation of Threema IDs on self-compiled apps. If you would like to use a self-compiled app, please restore the backup of an existing Threema ID. You can create Threema IDs and backups thereof using the purchased app.
If you have questions about the use of self-compiled apps or the license in general, feel free to contact us. We are publishing the source code in good faith, with transparency being the main goal. By having users pay for the development of the app, we can ensure that our goals sustainably align with the goals of our users: Great privacy and security, no ads, no collection of user data!